The experiments are designed primarily to test a set of hypotheses developed to explain the increased longevity of irradiated insects, but which have important implications for theories of mammalian aging. The central thesis is that some kinds of DNA-repair processes decline after differentiation of tissues to the postmitotic state, but that tolerated amounts of DNA damage serve as inducers to increase repair capacity. Predicted consequences which will be tested are that irradiation early in adult life of insects would retard decline in various manifestations of vigor, particularly ability to resist or to adapt to stresses in those cases for which adaptation would be expected to require transcription of large segments of the genome; that irradiation would lead to more rapid kinetics of repair and/or greater degree of repair in postmitotic tissues; and that other treatments which can damage DNA (e.g., alkylating agents) would also increase longevity. In testing these hypotheses, the most frequently-cited explanation for radiation-enhanced longevity, viz., decreased reproductive activity, will be critically evaluated. This evaluation will compare effects on increased longevity and on several parameters of reproductive activity of chemical agents, nonionizing radiations, and of modified exposure to ionizing radiation, such as LET, dose-protraction, and anaerobic exposure.